Al-Qaeda leader Ayman al-Zawahiri has released a video message warning the United States that it will face the “gravest consequences” if Boston Marathon bomber Dzhokhar Tsarnaev is executed. Tsarnaev, 21, was sentenced to death by lethal injection last year.

In fact, Zawahiri ordered the U.S. to avoid executing any Muslim prisoners.

“If the U.S. administration kills our brother, the hero Dzhokhar Tsarnaev, or any Muslim, it will bring America’s nationals the gravest consequences,” he said in the video, as translated by Reuters.

Zawahiri also declared that Western nations are “criminals, and they only understand the language of force.”

He urged Muslims to take as many Western hostages as they could, particularly from nations that were part of what he called, “the Crusaders’ campaign led by the United States.” He said those Western hostages could then be exchanged for Muslim prisoners.

The UK Daily Mailposted a clip from the video, which runs for an hour in its entirety:

The UK Guardianoffers a concise profile of the Egyptian-born Zawahiri, who is currently believed to be lurking somewhere along the border between Afghanistan and Pakistan:

Zawahiri took command of al-Qaida after Osama bin Laden was killed in 2011, by a US raid in Pakistan. The 64-year-old successor has eluded American strikes in the five years since, but the group’s influence has waned while rivals have gained power around the Middle East. Al-Qaida remains an active presence in Yemen, Pakistan and Libya, but its affiliate in Iraq grew into the terror group Isis as the civil war in neighboring Syria spiraled out of control.

Al-Qaida’s propaganda arm periodically threatens nations with recorded messages and long statements. In January Zawahiri condemned the execution of a Shiite cleric, Nimr al-Nimr, and 46 others by Saudi Arabia. Later that month two men attacked a Shia mosque in the majority Sunni country, though no group claimed credit. The attack resembled those inspired by Isis, whose leader Zawahiri has called a false “caliph of surprises”.

Zawahiri remains wanted by the US and has appeared in dozens of al-Qaida propaganda messages since 2003. In a September message he urged militants to move “the war to the heart of the homes and cities of the crusader west and specifically America”, and again called Isis’s territorial claims illegitimate.

The Long War Journalreported in June that Zawahiri had sworn allegiance to Mullah Haibatullah Akhundzada, the new leader of the Taliban.